914-SHO Motor teardown & inspection

[3d914]

I was looking at the oil rings more closely and noticed they fit tight and won't rotate around the piston easily. They're not supposed to be as loose as the compression rings are they?

Well, that's a pointless question since I just finished checking the ring gap on Cyls 1-3:

• Cyl #1/Ring #1 = .021
• Cyl #1/Ring #2 = .026
• Cyl #2/Ring #1 = .023
• Cyl #2/Ring #2 = .029
• Cyl #3/Ring #1 = .028
• Cyl #3/Ring #2 = ? Broke removing it

These aren't even close. Looks like I'm gong shopping again!
Any particular brand preferred?

 

[Off Road SHO]

Uh Oh, time to do a taper measurement. Looks like you might have too much wear on the cylinders.

Tom

 

[rubydist]

the ring gap will increase as the outside of the ring wears with use. the numbers you reported are not unusual or unexpected for an engine with the miles yours has.

before you install new rings, measure them, because too tight of gap is way worse than too loose of gap.

the oil rings will typically get stuck with carbon before the compression rings do, so I expected your oil rings to be sticky.

 

[3d914]

Planning to do the taper measurement as Tom suggested.

Any particular brand of rings preferred?

 

[intimdatr]

IIRC no. Most people use what they can source or prefer as a brand. I have never heard anyone having issue with a certain brand or anything of that nature. But i havent been here near the time of many others.

 

[Flintstone Chaz]

The smaller gap for the top ring vs. second ring seems odd. IIRC other engines I've built had a larger gap on the top ring. The top ring runs hotter.....more room for expansion would make sense to me.

Thoughts?

Charlie

 

[Off Road SHO]

I'm sure I'll get a lot of flack for this but, I've always preferred regular old cast iron rings over the chromium. Chrome is so hard it takes forever for them to seat. With the steel, 200 miles of spirited driving at varying RPM's and your done. JMHO.

Tom

 

[rubydist]

the ring end gap spec quoted in post #79 is for the 3.2.

for the 3.0, both compression rings are spec'd at .012-.018"

I would think the top one should be a little larger than the second one too, but they didn't ask me before they created the spec.

btw, for boosted applications, I would run at least .018" for the top one - you don't want that ring gap to close up under high load as that can result in the destruction of the top ring land. (I think the engine I just rebuilt for the RX7 has gaps in the .020" range for the top rings, iirc.)

 

[3d914]

Since my micrometer is on order, I decided to use the ring gap as a gauge for preliminary cylinder bore taper measurements. Took the top ring from piston #1 to use for all cylinders in three locations along each bore. I was also able to inspect the cylinders better with some daylight. I don't see any traces of cross-hatching in any of the cylinders (pics included).

Here's the preliminary taper results and pics.
Cylinder #1:

• Position-A = .022"
• Position-B = .0215
• Position-C = .0215

 

[3d914]

Cylinder #2:

• Position-A = .023
• Position-B = .022
• Position-C = .022

 

[3d914]

Cylinder #3:

• Position-A = .023
• Position-B = .022
• Position-C = .022

 

[3d914]

Cylinder #4:

• Position-A = .023
• Position-B = .022
• Position-C = .022

 

[3d914]

Cylinder #5:

• Position-A = .023
• Position-B = .0215
• Position-C = .022

 

[3d914]

Cylinder #6:

• Position-A = .023
• Position-B = .022
• Position-C = .022

 

[Flintstone Chaz]

I did some reading on ring end gaps. I was right that the top ring runs hotter and closes the gap more. However, there are other things going on. It seems that the larger second gap assures that gasses that get by the top ring can get by the second ring as well. That way the pressure below the top ring is never greater than the pressure above. This prevents ring futter.

The rule of thumb seems to be .004/inch of bore for the top ring and .005 for the second. That calculates as .014 top and .020 second for the 3.2. Right in line with factory specs......

I learn something every day!
Charlie

 

[rubydist]

I would hone those cylinders some to try to clean up the vertical scratch marks.

 

[3d914]

I would hone those cylinders some to try to clean up the vertical scratch marks.

rubydist, that was the plan - assuming they measure within specs. I have a micrometer on order, so should get to it here inthe next week or so.

 

[SHOVNST]

Does your fingernail catch in those scratches?

 

[Phoenix]

A very very light hone is recommended. Ive done both of mines , A pro engine guy came over my garage to perform it. He had the little rocks and the honing thing. He flooded everthing in wd40 while doing it. He also told me to rince/clean it with wd40 when ready to install pistons. He said it removes crap more then other products...

Then slap in new hasting rings and rev that thing to 8k without a puff or oil consumption.

Dont overbore.

 

[3d914]

Dont overbore.

phoenix, you're saying don't overbore - wouldn't that depend on whether the cylinders ID's are out of spec?